Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OVERCURRENT TRIP UNIT WITH INDICATION
OF DEPENDENCY OF TRIP FUNCTIONS
Field of the Invention:
This invention relates to overcurrent trip units for electric circuit
interrupting
devices, and more particularly to trip units which provide multiple protection
functions having a dependency relationship between selectable trip parameters,
and
which have a graphical representation of the current/time trip characteristic
of the
device displayed on an operator's panel.
Circuit interrupters for industrial and commercial applications commonly
provide multiple protection functions. For instance, such circuit interrupters
typically
provide instantaneous and long delay protection and may also provide short
delay
protection and/or ground fault protection. The instantaneous trip function
protects 'the
electrical conductors to which the current interrupting device is connected
from
damaging overcurrent conditions, such as short circuits. The long delay trip
function
protects the load served by the protected electrical system from overcurrents.
The
short delay trip function can be used to coordinate tripping of down stream
circuit
interrupters in a hierarchy of circuit interrupters. As implied, the ground
fault trip
function protects the electrical system from faults to ground.
Each circuit interrupter is designed for a specific maximum continuous
current. This
current rating may be set by selection of a resistor which converts the
current to a
voltage for use by the trip unit. In some cases, a single circuit interrupter
may
be easily adapted for installations which call for a range of maximum
continuous
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currents, up to the design limits of the frame, through use of a removable
rating plug
by which the current rating of the device can be established. Traditionally,
the pick-up
currents for the various protection functions have been selectable multiples
or
fractions of this current rating. Thus, instantaneous protection trips the
device
anytime the current reaches a selected multiple of the rated current, such as
for
example, ten times the rated current. Pick-up for short delay protection is a
lesser
multiple of the rated current, while pick-up current for long delay protection
may be a
fraction of the rated current.
The long delay and short delay trip functions require that the overcurrent
conditions exist for a period of time. Typically, the long delay trip function
has
been selected such that a trip signal is generated if the current exceeds six
times the
rated current for the selected long delay interval. As damage can also occur
at lesser
current levels for longer periods of time, an inverse time function is used
for long
delay protection. Thus, the smaller the current the longer the time to trip,
and the
larger the current, the shorter the time to trip. Typically, an 12t
characteristic has been
used for long delay protection, altl-~ough in some circuit breakers the
inverse time
function is programmable, and may include other functions such as It or 14t.
Typically, the short delay trip is only generated when the short delay pick-up
current
is exceeded for the short delay interval, although in some applications an
inverse time
function is used for short delay protection also.
If the current/time characteristic of a circuit interrupter is plotted on a
logarithmic scale with current on the abscissa and time on the ordinate, the
pick-up
currents appear as vertical line segments, and the Ilt characteristic of the
long delay,
and if used for the short delay, appear as straight diagonal lines. The
assignee of this
application manufactures circuit breakers in which a graphic representation of
this
response is provided on the operator's panel together with switches for
setting the
parameters of the protection functions located
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adjacent the appropriate section of the current/time characteristic. Such a
circuit
breaker is disclosed in U.S. Patent No. 4,752,853.
With the traditional approach of having the instantaneous, short delay
and long delay pick-up currents selectable as factors of the rated current,
the
S_ protection functions are independent of one another, so that setting the
parameters
for one function does not affect the setting of parameters for another
function.
However, it has been found desirable in some applications to adjust the short
delay
trip function in coordination with the long delay trip function. Thus, trip
units have
been designed in which the short delay pick-up current is changed to a
multiple of
the long delay pick-up current, and not the rated current. This results in
dependence of the short delay protection upon the long delay pick-up level.
There is a need to communicate this dependence of the short delay
upon the long delay pick-up level and the adjustment of the inverse time
function
of the circuit interrupting device to the user.
1~ &IT~IMARY OF THE INVENTION
This need and others are satisfied by the invention which is directed
to an overcurrent trip unit for an electric current interrupting device which
has
means establishing a current rating for the current interrupting device, such
as, for
example, a rating plug, means sensing current in the device, means responsive
to
the sensed current for generating a trip signal in accordance with a
current/time
characteristic having a first portion being a function of the current rating
which is independent of a second function, and a second portion having a
plurality of sections at least some of which are functions of the current
rating which are dependent on one another. The trip unit of the
invention further has an operator's panel having a graphical
representation of the current/time characteristic with a first indicia
indicating the first portion of the current/time characteristic and
a second indicia indicating the second portion of the current/time
characteristic.
The different indicia for the two portions of the graphic representation of
the
current/time characteristic, which are preferably different colors, make the
operator
aware that adjustment of a parameter affecting a trip function represented by
the
interdependent portion of the current/time characteristic will affect another
parameter.
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A full understanding of the invention can be gained from the
following description of the preferred embodiments when read in conjunction
with
the accompanying drawings in which:
Figure 1 is a schematic diagram in block form of a circuit interrupter
in accordance with the invention shown connected to an electrical system.
Figure 2 is a logarithmic plot of the currentltime characteristic of the
circuit breaker of the invention.
Figure 3 is an illustration of the operator's panel of the circuit breaker
1~ of Figure 1 in accordance with a first embodiment of the invention.
Figure 4 is an illustration of the operator's panel of the circuit breaker
of Figure 1 in accordance with another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The circuit interrupting device in accordance with the preferred
embodiment of the invention is the circuit breaker shown in Figure 1 connected
for
protection of an electrical system 1 which includes three phase conductors 3a,
3b
and 3c, and which may also include a neutral conductor 3n. The circuit
interrupter
in the preferred embodiment of the invention is a microprocessor based circuit
breaker 5. The circuit breaker 5 has current transformers 7a, 7b, 7c, and 7n
which
2~( generate signals representative of the currents flowing in the respective
phase
conductors, and in the neutral conductor if desired. An electronic trip unit 9
monitors the currents sensed by the current transformers and generates a trip
signal
in response to predetermined current/time conditions. The electronic trip unit
9
incorporates a microprocessor 11. The microprocessor based electronic trip
unit 11
may be of the type disclosed in U.S. Patent No. 4,752,853. The electronic
trip unit 9 has a removable rating plug 13. The rating plug 13
incorporates a resistor which can be selected to set the maximum
continuous current that will be permitted by the circuit breaker 5 as
previously discussed.
~Q The electronic trip unit 9 generates a trip signal in response to the
specified overcurrent conditions. This trip signal actuates a trip device 15
which
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opens sets of separable contacts 17a, 17b and 17c, to interrupt current
through the
phase conductors of the electrical system 1.
The circuit breaker 5 provides several modes of protection of the
types previously discussed. In particular, long delay and instantaneous
protection
are provided. Short delay and ground fault protection can also be provided.
These
various protection functions can be more fully understood by reference to
Figure 2
which illustrates the current/time characteristic 19 of the electronic trip
unit 9. This
is a log-log plot with current shown on the abscissa and time on the ordinate.
The instantaneous protection function is represented in Figure 2 by
the portion 21 of the current/time characteristic 19. The instantaneous trip
function
protects the circuit breaker from large overcurrents such as short circuits.
If the
current exceeds the pick-up current for the instantaneous trip function, the
circuit
breaker trips immediately.
The second portion 23 of the currentltime characteristic 19 above the
instantaneous portion 21 provides the time delayed trip functions. A first
section
25 of this upper portion of the currentltime characteristic provides the short
delay
trip function. The vertical line segment 27 is the short delay pick-up
current. As
can be seen from Figure 2, a current which exceeds the short delay pick-up
current
27 must persist for a period of time determined by the short delay pick-up
time 29
2Q before the circuit breaker will trip. In some applications an inverse time
function
is used for the short delay trip. This inverse time function for short delay
protection
is indicated by the dashed line 31. With such an inverse time function for the
short
delay it can be seen from Figure 2 that a current above the short delay pick-
up value
27 would have to persist for a longer period of time before the circuit
breaker trips,
but that at higher current levels the circuit breaker would trip sooner.
A second section 33 of the upper portion 25 of the current/time
characteristic 19 provides the long delay protection. The long delay pick-up
current
is represented by the vertical line segment 35. Commonly, the long delay trip
function is provided with an inverse time function represented by the diagonal
line
3~ segment 37. Typically 12t inverse time functions are used for the long
delay and
if used, the short delay trip functions. However, other inverse time functions
such
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as It or l~t may be used. These other inverse time functions would provide a
different slope to the diagonal line segments 31 and 37.
The instantaneous pick-up current 21, short delay pick-up current 27,
and long delay pick-up current 35 are selectable. The instantaneous pick-up
current
21 is a selected multiple of the circuit breaker rated current, h , which as
discussed
above is set by the rating plug 13. The long delay pick-up current, 1,, shown
at 35
is a selectable fraction of the rated current, l~ . The short delay pick-up
current 27
is a selectable multiple of the long delay pick-up current I~ shown at 35. The
long
delay time 39 establishes the point 41 on the current/time characteristic 19.
The
_1Q current at point 41, which must persist for the long delay time 39 in
order to
generate the long delay trip is six times the long delay pick-up current 35 in
the
exemplary circuit breaker.
As the short delay pick-up current 27 is a multiple of the long delay
pick-up current 35, a change in the value of the long delay pick-up current
15 automatically effects a change in the short delay pick-up current. Thus as
shown
in Figure 2, a reduction in the long delay pick-up current to the current 35~,
shifts
the short delay pick-up current to the current 27~. In addition, since the
long delay
trip is generated in response to a current equal to six times the long delay
pick-up
current I, which has shifted to 35~, the inverse time function shifts to the
position
37~. A similar shift would occur in the short delay inverse time function if
utilized.
Thus, it can be seen that a change in the long delay pick-up current results
in a shift
of the entire upper portion 25~ of the current/time function 19. As the
instantaneous
trip function is a function of the rated current l,~ , it is not affected by
the change in
long delay pick-up current. It is clear from the above, that the instantaneous
trip
function is independent of the long delay pick-up function.
For purposes of illustration, in other types of circuit breakers, the
short delay pick-up current 27 is a function of the rated current 1~ rather
than the
long delay pick-up current 35. Also, in those circuit breakers, the long delay
is set
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to trip upon the expiration of the long delay time interval at a current equal
to six
times the rated current, again h , rather than the long delay pick-up current
l, .
Hence, a change in the long delay pick-up current in those circuit
breakers to, for instance the current 35~~ as shown in Figure 2 by the chain
line,
does not affect either the short delay trip function 25 or the long delay
inverse time
function 37. It does, however, result in an extension, or shortening, of the
inverse
time function 37, as represented by the intersection of the line segment 37
and the
line segment 35~~ in Figure 2.
An important feature of the present invention is the operator interface.
1~( The purpose of this interface is to communicate to the operator as clearly
as possible
the capabilities of the circuit breaker 5, and the operating status of the
circuit
breaker, and to provide easily understood and operated means for setting the
various
trip functions of the circuit breaker.
The operator interface for the circuit breaker 5 includes an operator
15 panel mounted on the trip unit 9. Figure 3 illustrates the relevant portion
of the
operator's panel 43. A prominent feature of this operator panel is a graphical
representation 45 of the current/time characteristic 19 illustrated in Figure
2. A
number of rotary switches are mounted on the operator's panel 43 for setting
the
parameters for the various trip functions of the circuit breaker. These
switches are
placed on the panel 43 adjacent the pertinent portion of the representation 45
of the
current/time characteristic. Thus, a switch 47 adjacent the long delay pick-up
current section 49 of the characteristic 43 provides for selection of the long
delay
pick-up current. The long delay pick-up current is selected as a function of
the
rated current In . The fraction selected by the switch 47 is visually
presented in a
display window S 1 adjacent to switch 47. The legend "X 1,~= Ir " appears next
to
the window 51 to inform the operator that the long delay pick-up current
settingl,
is a function of 1~. Adjacent to section 81 on the characteristic 43
indicating the
long delay time setting, is a switch 55 for selecting the long delay time
interval, and
a window 57 for displaying the selected time setting. A legend adjacent the
switch
3Q 55 and window 57 indicates that this is the long delay time setting and
that the long
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delay trip will occur at the selected time at six times the long delay pick-up
setting
Ir .
Another rotary switch 59 adjacent the short delay pick-up current 61
of the characteristic 45 is for selection of the short delay setting which is
displayed
in the window 63. An adjacent legend reminds the operator that the short delay
setting is the factor displayed in the window 63 multiplied by Il, the long
delay
setting. The short delay time is set by the switch 65 and displayed in the
window
67. The pick-up current for the instantaneous trip is set by the switch 69,
and the
factor by which the rated current is multiplied to set the instantaneous trip
is
displayed in the window 71. Both of which are adjacent the instantaneous trip
current portion 73 of the characteristic 45.
As explained in connection with Figure 2, the lower portion 75 of the
current/time characteristic 45 on the operator's panel as shown in Figure 3
represents the instantaneous trip function which is independent of the other
trip
~S functions. The upper portion 77 of the representation 45 of the
current/trip
characteristic includes a section 79 displaying the short delay trip function
and a
section 81 displaying the long delay trip function. In order to emphasize to
the
operator that the short delay function is dependent upon the long delay pick-
up level
while the instantaneous function is independent, a first indicia 83 is used
for the
lower portion 75 of the characteristic 45 while a second indicia 85 is used
for the
upper portion 77. In the preferred embodiment of the invention different
colors are
used as the indicia 83 and 85. In the exemplary circuit breaker 5, the color
white
is used as the indicia 83, and the color blue is used as the indicia 85.
Clearly, other
colors could be used to identify the independent and interdependent portions
of the
representation 45 of the currentltime characteristic, or even other forms of
indicia
could be used for this purpose.
The operator's panel also includes light emitting diodes (LEDs) which
are used to inform the operator of the operating state of the circuit breaker
5. The
LED 87, which is embedded in the representation 45 of the current/time
~Q characteristic at the location of the instantaneous pick-up current value
73, is
illuminated in response to an instantaneous trip. The LED 89 adjacent the
short
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delay pick-up current portion 61 of the representation 45 is illuminated
continuously
in response to a short delay trip. LED 91 flashes when the long delay pick-up
current is exceeded and comes on steady following a long delay trip. The
operator's
panel 43 of the circuit breaker 5 also includes a switch 93 for setting a test
pick-up
current for the trip unit. This test current is set as a factor of the rated
currenth
which is displayed in the window 95. The test is initiated by pressing a test
button
97. The circuit breaker is reset following a trip by pressing a trip reset
button 99.
An LED 101 blinks continuously when the circuit breaker is operating normally
to
provide an indication of the operating status of the breaker.
Various combinations of protection functions can be provided by the
circuit breaker 5. Figure 4 illustrates the operator's panel 43~ for another
embodiment of the invention in which long delay and instantaneous protection
are
provided, but not short delay protection, and in which ground fault protection
is
provided. Elements of the modified operator's panel 43~ which are the same as
15 those in the operator's panel 43 shown in Figure 2 are identified with like
reference
characters. Elements which are similar to elements of the panel 43 are
identified
by primed reference characters. As can be seen from Figure 4, the
representation45~
of the current/time characteristic does not include the short delay function,
but the
upper portion representing the long delay function is marked with the
indicia85~
20 to indicate that the inverse time function is shifted with a change in the
long delay
pick-up setting.
The operator's panel 43~ includes a graphical representation 103 of
the ground fault trip function. The ground fault trip current is a fraction of
the
rated current In , which is set by the switch 105 and is displayed in the
window 107.
2_~ The time for the ground fault trip is set by the switch 109, and displayed
in the
window 111. The ground fault trip function may be an instantaneous trip as
indicated by the graphical representation 103 or an inverse time function as
represented by the indicia 113 shown in dashed line.
Circuit breakers in accordance with the invention may use various
30 combinations of the trip functions illustrated in the embodiments shown in
Figures
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3 and 4. For instance, a circuit breaker could incorporate all of the trip
functions:
long delay, short delay, instantaneous and ground fault.
While specific embodiments of the invention have been described in detail, it
will be appreciated by those skilled in the art that various modifications,
and
alternatives to those details could be developed in light of the overall
teachings of the
disclosure. Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of invention which is to be
given the
full breadth of the appended claims and any and all equivalents thereof.
